The present invention relates generally to the field of integrated circuit (IC) fabrication and, more particularly, to the a ferroelectric film of multi-phase lead germanium oxide (PGO), and method of depositing the multi-phase PGO film through a metal organic chemical vapor deposition (MOCVD) process.
Ferroelectric films have attracted great interest in recent years because of their applications in electro-optics, pyroelectric devices, frequency agile electronics, and non-volatile memory devices. The fabrication and characterization of ferroelectric lead germanium oxide thin films, especially Pb.sub.5 Ge.sub.3 O.sub.11, are of current interest. Lead germanate (Pb.sub.5 Ge.sub.3 O.sub.11) is a relative new member of ferroelectric family. The ferroelectricity in this material was first discovered by Iwasaki et al., Appl. Phys. Lett. 18, 444, 1971. The piezoelectric, dielectric and electric-optics properties of single crystal and polycrystalline materials have been well reported in the literature. The non-perovskite uniaxial ferroelectric Pb.sub.5 Ge.sub.3 O.sub.11, with polar direction parallel to the c-axis, belongs to the trigonal space group P3 at room temperature. This material transforms to the hexagonal, space group P6 (=P3/m) paraelectric phase above the Curie temperature (Tc=178.degree. C.). Since this uniaxial ferroelectric Pb.sub.5 Ge.sub.3 O.sub.11 possesses only 180.degree. domains, there are no ferroelastic effects that tend to reorient domains through 90.degree., relaxing the polarization.
Another interesting feature of this materials is that Pb.sub.5 Ge.sub.3 O.sub.11 has small dielectric constant and small remnant polarization which are also suitable for ferroelectric non-volatile memory devices such as metal ferroelectric metal oxide silicon (MFMOS), metal ferroelectric metal silicon (MFMS), metal ferroelectric insulator silicon (MFIS), metal insulator ferroelectric silicon (MIFS), metal insulator ferroelectric insulator silicon (MIFIS), and metal ferroelectric silicon (MFS) type memories, especially in one transistor memory applications. Pb.sub.5 Ge.sub.3 O.sub.11 also has some potential for thermal detector applications because of its pyroelectric and dielectric characteristics.
The thin films of lead germanate have been made by thermal evaporation and flash evaporation (A. Mansingh et al., J. Appl. Phys. 51, 5408, 1980), dc reactive sputtering (H. Schmitt et al., Ferroelectronics 56, 141, 1984), laser ablation (S. B. Krupanidhi et al., Proceedings of 3.sup.rd International Symp. on Integrated Ferroelectronics, 100, 1991), and sol-gel technique (J. J. Lee et al., Appl. Phys. Lett. 60, 827, 1992).
Single crystal of Pb.sub.5 Ge.sub.3 O.sub.11 has been shown to have spontaneous polarization and coercive field of 4 micro-coulombs per centimeter squared (.mu.C/cm.sup.2) and 14 kilovolts per centimeter (kV/cm) in the direction along the c-axis, respectively. The c-axis oriented Pb.sub.5 Ge.sub.3 O.sub.11 thin films exhibited poor ferroelectric properties: lower polarization (2-3 .mu.C/cm.sup.2), higher coercive field (55-135 kV/cm), and their hysteresis loops were not saturated and square. In order to switch the PGO ferroelectric domains, a very high operation voltage must be used, which precludes their use in the memory devices.
The present invention focuses on improving ferroelectric properties by the combination of two phases of PGO film. In co-pending patent application Ser. No. 09/301,420, entitled "C-Axis Oriented Lead Germanate Film and Deposition Method for Same", invented by Tingkai Li et al., filed on Apr. 28, 1999, the Pb.sub.5 Ge.sub.3 O.sub.11, film is crystallographically oriented in the c-axis. This film has increased Pr and dielectric constant values, and is useful in making one transistor (1T) memories applications.
In co-pending patent application Ser. No. 09/302,272, entitled "Epitaxially Grown Lead Germanate Film and Deposition Method", invented by Tingkai Li et al., filed on Apr. 28, 1999, an appropriate content of the second phase Pb.sub.3 GeO.sub.5 is added to Pb.sub.5 Ge.sub.3 O.sub.11, forming large grain sizes with extremely high C-axis orientation. As a result, high Pr and Ec values, as well as lower dielectric constant, is obtained. This film is useful in 1T, one transistor/one capacitor (1T/1C) FeRAM memory devices.
In co-pending patent application Ser. No. 09/301,434, entitled "Ferroelastic Lead Germanate Film and Deposition Method", invented by Tingkai Li et al., filed on Apr. 28, 1999, a CVD Pb.sub.5 Ge.sub.3 O.sub.11 film is formed having improved ferroelastic properties. This film is useful in microelectromechanical systems (MEMS), high speed multichip module (MCM), DRAM, and FeRAM applications. The above-mentioned co-pending patent application are incorporated herein by reference.
It would be advantageous if a process could be developed for Lead Germanium Oxide thin film materials to improve their ferroelectric properties, making these film useable as ferroelectric PGO capacitors.
It would be advantageous if c-axis orientation and grain growth could be promoted in Pb.sub.5 Ge.sub.3 O.sub.11 thin films, to form PGO films having a higher polarization and lower coercive field.
It would be advantageous if a method of using a second phase of PGO to improve the ferroelectric properties of c-axis oriented Pb.sub.5 Ge.sub.3 O.sub.11 for one-transistor memory applications could be developed.
Accordingly, a method for forming a lead germanium oxide (PGO) film on an integrated circuit (IC) wafer has been provided. The method comprises the steps of:
a) mixing [Pb(thd).sub.2 ] and [Ge(ETO).sub.4 ] to form a PGO mixture having a molar ratio of approximately 5:3, for example between 5:3 and 5.3:3; PA1 b) dissolving the mixture of Step a) with a solvent of tetrahydrofuran, isopropanol, and tetraglyme, having a molar ration of 8:2:1, to form a precursor solution having a concentration of approximately 0.1 to 0.3 moles of PGO mixture per liter of solvent; PA1 c) introducing the precursor solution of Step b) at a rate in the range of approximately 0.1 to 0.5 milliliters per minute (ml/min), and heating the solution to a temperature in the range of approximately 140 to 250 degrees C, creating a precursor gas; PA1 d) introducing the precursor vapor to the IC wafer; and PA1 e) decomposing the precursor gas formed in Step c) on the wafer at a temperature in the range of approximately 400 to 650 degrees C to form a PGO film, having ferroelectric properties, including a first phase of Pb.sub.5 Ge.sub.3 O.sub.11 and a second phase of Pb.sub.3 GeO.sub.5. In this manner, the ferroelectric characteristics of the first phase are improved by adding the second phase. PA1 d.sub.1) heating the IC wafer chuck to a temperature in the range of approximately 400 to 500 degrees C, and decomposing precursor gas on the IC wafer to form a first thickness nucleation layer of about 100 to 600 .ANG. of PGO film with a first phase of Pb.sub.5 Ge.sub.3 O.sub.11 ; and PA1 in which Step e) includes heating the wafer chuck to a temperature in the range of approximately 500 to 700 degrees C, and decomposing precursor gas on the IC wafer to form a second thickness of about 1000-3000 .ANG.. PA1 f) rapid thermal annealing (RTA) the PGO film formed in Step e) at a temperature in the range of approximately 500 to 750 degrees C in an atmosphere selected from the group of oxygen and oxygen with Pb atmospheres, whereby the ferroelectric properties of the PGO film are improved; PA1 g) forming a conductive electrode overlying the PGO film formed in Step e); and PA1 h) rapid thermal annealing the PGO film formed in Step e) at a temperature in the range of approximately 500 to 750 degrees C in an atmosphere selected from the group of oxygen, and oxygen with Pb atmospheres, whereby the interface between the PGO film, formed in Steps d) and e), and the electrode formed in Step g), is improved.
In some aspects of the invention, Step a) includes mixing the [Pb(thd).sub.2 ] and [Ge(ETO).sub.4 ] in a molar ratio in the range of approximately 5.1:3 to 5.2:3, whereby the additional [Pb(thd).sub.2 ] promotes improved ferroelectric properties in the PGO film.
In another embodiment, the IC wafer is located on a wafer chuck in a vacuum chamber or reactor, and includes a further step, following Step d), and preceding Step e), of:
Further steps follow Step e) of:
The method of the present invention is suitable for the formation of a lead germanium oxide (PGO) film having ferroelectric properties. The PGO film formed comprises a first phase of Pb.sub.5 Ge.sub.3 O.sub.11 and a second phase of Pb.sub.3 GeO.sub.5, whereby the Pb.sub.3 GeO.sub.5 phase promotes improved ferroelectric properties in the PGO film. The phase range for the Pb.sub.3 GeO.sub.5 second phase is in the range of approximately 1 to 5%, and the first Pb.sub.5 Ge.sub.3 O.sub.11 phase has a grain size in the range of approximately 0.3 to 2 microns. In one aspect of the invention, the second Pb.sub.3 GeO.sub.5 phase is homogeneously distributed in the first Pb.sub.5 Ge.sub.3 O.sub.11 phase.
The method of the present invention is also suitable for providing a capacitor having ferroelectric properties. The capacitor provided comprises a first conductive electrode, a PGO film including a first phase of Pb.sub.5 Ge.sub.3 O.sub.11 and a second phase of Pb.sub.3 GeO.sub.5 overlying the first electrode, and a second conductive electrode overlying the PGO film, whereby a PGO film capacitor is formed. The ferroelectric properties include a dielectric constant in the range of approximately 30 to 100.